The exemplary embodiments relate generally to gas turbine engine components and more specifically to leaf seal assemblies for turbine nozzle assemblies.
Gas turbine engines typically include a compressor, a combustor, and at least one turbine. The compressor may compress air, which may be mixed with fuel and channeled to the combustor. The mixture may then be ignited for generating hot combustion gases, and the combustion gases may be channeled to the turbine. The turbine may extract energy from the combustion gases for powering the compressor, as well as producing useful work to propel an aircraft in flight or to power a load, such as an electrical generator.
The turbine may include a stator assembly and a rotor assembly. The stator assembly may include a stationary nozzle assembly having a plurality of circumferentially spaced apart airfoils extending radially between inner and outer bands, which define a flow path for channeling combustion gases therethrough. Typically the airfoils and bands are formed into a plurality of segments, which may include one (typically called a singlet) or two spaced apart airfoils radially extending between an inner and an outer band. The segments are joined together to form the nozzle assembly.
The rotor assembly may be downstream of the stator assembly and may include a plurality of blades extending radially outward from a disk. Each rotor blade may include an airfoil, which may extend between a platform and a tip. Each rotor blade may also include a root that may extend below the platform and be received in a corresponding slot in the disk. Alternatively, the disk may be a blisk or bladed disk, which may alleviate the need for a root and the airfoil may extend directly from the disk. The rotor assembly may be bounded radially at the tip by a stationary annular shroud. The shrouds and platforms (or disk, in the case of a blisk) define a flow path for channeling the combustion gases therethrough. The nozzles and shrouds are separately manufactured and assembled into the engine. Accordingly, gaps are necessarily provided therebetween for both assembly purposes as well as for accommodating differential thermal expansion and contraction during operation of the engine.
The gaps between the stationary components are suitably sealed for preventing leakage therethrough. In a typical turbine nozzle, a portion of air is bled from the compressor and channeled through the nozzles for cooling thereof. The use of bleed air reduces the overall efficiency of the engine and, therefore, is minimized whenever possible. The bleed air is at a relatively high pressure, which is greater than the pressure of the combustion gases flowing through the turbine nozzle. As such, the bleed air would leak into the flow path if suitable seals were not provided between the stationary components.
A typical seal used to seal these gaps is a leaf seal. A typical leaf seal is arcuate and disposed end to end around the circumference of the stator components. For example, the radially outer band of the nozzle includes axially spaced apart forward and aft rails. The rails extend radially outwardly and abut a complementary surface of an adjoining structural component, such as, but not limited to, a shroud, a shroud hanger, and/or a combustor liner, for providing a primary friction seal therewith. The leaf seal provides a secondary seal at this junction and bridges a portion of the rail and the adjoining structural component. Leaf seals are typically relatively thin, compliant sections, which are adapted to slide along a pin fixed to one of the adjoining structural components.
Regardless of the particular shape of the structural components to be sealed, leaf seals are movable to a closed, sealing position in which they engage each structural component and seal the space therebetween, and an open position in which at least one portion of the leaf seals disengage a structural component and allow the passage of gases in between such components. In most applications, movement of the leaf seals along the pins to a closed position is affected by applying a pressure differential across seal, i.e., relatively high pressure on one side of the seal and comparatively low pressure on the opposite side thereof forces the seal to a closed, sealed position against surfaces of the adjoining structural components to prevent the passage of gases therebetween.
While leaf seals have found widespread use in turbine engines, their effectiveness in creating a fluid tight seal is dependent on the presence of a sufficient pressure differential between one side of the seal and the other. During certain operating stages of a turbine engine, the difference in fluid pressure on opposite sides of the leaf seals is relatively low. Under these conditions, it is possible for the leaf seals to unseat from their engagement with the abutting structural components of the turbo machine and allow leakage therebetween. A relatively small pressure differential across the leaf seals also permits movement or vibration of the leaf seals with respect to the structural components that they contact. This vibration of the leaf seals, which is caused by operation of the turbine engine and other sources, creates undesirable wear both of the leaf seals and the surfaces of the structural components against which the leaf seals rest. Such wear not only results in leakage of gases between the leaf seals and structural components of the turbine engine, but can cause premature failure thereof.
To overcome this problem, other designs have included a biasing structure, such as a spring, to bias the leaf seal toward a certain position. For example, a band may have two circumferentially spaced apart, radially extending tabs spaced axially from a rail. A recess may be formed between the tabs and the rail where the leaf seal and spring are disposed. The tabs, leaf seals and springs may include holes for receiving a pin for mounting to the band. At least one of the tabs is typically spaced apart from the circumferential edges of the band. The tab, leaf seal and spring are arranged so that the spring forces the leaf seal against an adjoining structural component so as to maintain the leaf seal in a closed, sealed position at all times.
In some instances, such as, but not limited to, low emissions combustors, this configuration is not sufficient. For example, low emissions combustors are susceptible to flame instability, which may lead to acoustic resonance and high dynamic pressure variation. The high frequency pressure fluctuations can damage the leaf seals, particularly the leaf seals between the aft edge of the combustor liner and the leading edge of the nozzle bands, by repeatedly loading and unloading the seals against the adjoining structural component. The seals are particularly susceptible to damage where they are unsupported by the springs and/or tabs. The seals may not be fully supported at their circumferential edges and/or between the tabs on the bands.